1. Field of the Invention
The present invention relates to a particulate matter concentration measuring apparatus for measuring a concentration of particulate matter in a sample gas, and specifically to a filter tape used for use in measuring the concentration of the particulate matter.
2. Description of Related Art
As an apparatus for measuring particulate matter (hereinafter, refer to as PM) in the atmosphere, there is an apparatus which continuously receives fixed flow rate samples of the atmosphere into a sampling tube (a gas introduction pipe) as a sample gas, continuously collects the PM from the sample gas within a vacuum chamber provided in a downstream side of the sampling tube by using a collecting means such as a filter tape (a ribbon filter) or the like, irradiates beta rays onto the collected PM from a beta-ray source, detects transmitted beta rays at that time by a detector, and measures a concentration of the collected PM in accordance with a beta-ray absorbing method by using an output of the detector.
This PM concentration measuring apparatus has a plurality of exhaust holes for discharging the sample gas passing through the filter tape, and is provided with a supporting means for supporting the filter tape in a state of preventing the filter tape from being deformed during collection of the sample. FIG. 7 shows a structure of a plate-like portion 60 held by a supporting means. The plate-like portion 60 is structured by bonding a thin plate-like portion to a hole portion provided in the supporting means, the plate-like portion 60 has three exhaust holes 61, 62 and 63, and is arranged adjacent to the under face side of a filter tape 64.
The sample gas is drawn by a sampling pump arranged in a under face side of the plate-like portion 60, and passes through the filter tape 64 from the upper face side to the under face side, and through three exhaust holes 61, 62 and 63. The sample gas is drawn through the filter tape and the plate-like portion for a fixed time (for example, one hour), whereby the collecting region is formed on the filter tape 64. Reference numeral 65 denotes a take-up direction of the filter tape 64.
In the case of measuring the concentration of the PM by using the beta-ray absorbing method, the beta rays are irradiated onto the collecting region from a radiation source arranged in a under face side of the plate-like portion 60, and the beta rays passing through the collecting region are detected by a detector arranged in the upper face side of the filter tape 64 via a protection film provided at the inlet of the detector, whereby the detector produces a signal representative of the concentration of the PM. In this case, the protection film has a function of restricting a pressure loss applied to the detector caused by the sample gas suction of the sampling pump.
Meanwhile, in the beta-ray absorbing method, it is desirable that the weight (the density) of the filter tape 64 be small in order to provide increased measurement sensitivity. However, the material of the filter tape 64 is generally glass fiber, and the fixed thickness (450 μm; average value) and a fixed weight (7 mg/cm2; average value) are required in the glass fiber for the purpose of obtaining enough strength to withstand continuous use. Accordingly, since the strength of the filter tape 64 cannot be preserved while reducing the weight (the density) of the glass fiber, such filter tape is undesirable for use in continuous measurement. Further, high sensitivity is difficult to obtain with glass fiber tape since the beta rays are partially absorbed by the glass fiber.
Further, since three exhaust holes 61, 62 and 63 are considerably large, some deformation of the filter tape 64 often occurs partially collapsing the filter tape 64 in these regions during collection of the PM. This deformation may result in slight differences in concentration measurements, making it difficult to obtain reproducible measurement results.
Further, in the PM concentration measuring apparatus of beta-ray absorbing type mentioned above, a proportional counter is generally used as a detector for detecting the transmitting beta rays. Typically, the proportional counter can detect alpha rays as well as beta rays. In the proportional counter, as is understood from transmission distribution curves A and B in FIG. 9, since peaks PA and PB in a transmission amount of the alpha rays (shown by the curve A in the drawing) and the beta rays (shown by the curve B in the drawing) are different, there is no problem in most of the spectrum in terms of detecting the alpha rays and the beta rays. However, in the portion shown by reference symbol C in FIG. 9, since the beta rays and the alpha rays overlap in this region, the alpha rays contribute to an error factor only to the degree they overlap the beta rays in the portion C.
Further, alpha rays (radon gas) and beta rays exist in nature in trace quantities. Accordingly, in the case of measuring the concentration of the PM collected in accordance with the beta-ray absorption method, any radioactive materials other than the beta-ray source (a sealed ray source) within the measuring apparatus may contribute to an error factor and make it difficult to accurately measure the PM in accordance with the beta-ray absorption detection method.
Meanwhile, it is desirable to measure even the minute PM having a particle diameter equal to or less than 2.5 μm (hereinafter, refer to as PM2.5) at a high sensitivity. However, since an error influence by the alpha rays with respect to the beta rays and an error influence by the beta rays existing in the nature generate a great obstacle in the case of measuring the PM2.5 at a high sensitivity, it is desired to make these influences as small as possible.
FIG. 15 shows a state in which a conventional PM concentration measuring apparatus 120 for measuring PM in a sample gas is in place. This PM concentration measuring apparatus 120 continuously draws in a fixed flow rate of sample gas into a sampling tube and continuously collects the PM within the sample gas S in a chamber provided in a downstream side of the sampling tube by using a collecting means, for example, a ribbon-like filter or the like, and measures the concentration of the collected PM in accordance with the beta-ray absorbing method.
The PM concentration measuring apparatus 120 is, for example, placed in a room, and an introduction port for the sample gas S is communicated and connected to a sampling pipe 102 constituted by a synthetic resin hose communicating with a sample gas introduction portion 102a, for example, provided in a rooftop portion, whereby it is possible to draw in the sample gas S in the open air and measure the PM concentration by the PM concentration measuring apparatus 120 in the room. Further, a cyclone type sampler is installed within the PM concentration measuring apparatus 120 so as to constitute a sizing device for collecting the PM contained in the sample gas S. In this case, the cyclone type sampler in the present specification corresponds to a sampling apparatus (a cyclone type volume sampler) for sizing the PM by using a centrifugal separation caused by an eddy current of the sample gas S, and may be simply referred to as a cyclone in the following description.
FIG. 16 shows an impact type sampler 121 corresponding to the sizing device which is designated as a standard in the U.S. and Europe and an example of a PM concentration measuring apparatus using the impact type sampler 121. In this case, the impact type sampler in the present specification indicates a suction sampler (an impact type low volume sampler) removing the PM having a large particle diameter on the basis of collision of the sample gas S and selectively sampling the PM having a small particle diameter. In the following description, this impact type sampler may be simply called as an impacter. This impacter 121 has a sizing device main body 105 collecting the PM having a large particle diameter equal to or more than 2.5 μm from all the PM removed from the sample gas S, and an introduction portion 122 for the sample gas S to the sizing device main body 105.
FIG. 17 is a view showing a structure of the introduction portion 122 in the impacter 121 mentioned above in an enlarged manner. In FIG. 17, reference numeral 109 denotes a funnel-shaped sample intake port portion formed in an upper end portion of the sizing device main body 105, reference numeral 110 denotes a mounting flange formed in the sample intake port portion 109, reference numeral 111 denotes a clip plate screwed in the mounting flange 110, for example, using threaded holes formed at an interval of 90 degrees, reference numeral 123 denotes a guide body mounted by a fixed interval to the clip plate 111 by using a spacer 124, and reference numeral 125 denotes an annular net-like body clamped between the guide body 123 and the clip plate and provided for the purpose of preventing insects or the like from being mixed. These members 111 to 125 form the introduction portion 122.
Meanwhile, in recent years, taking into consideration that the cyclone has a defect in a low collecting efficiency of the fine particles such as the PM2.5, the Japanese Environmental Agency issued a study entitled “Preliminary Manual for Method of Measuring Mass Concentration of Particulate Matter (PM2.5) in the Atmosphere” in September 2000, and preliminarily employing the impacter 121 for selectively sizing the fine particles having a diameter equal to or less than 2.5 μm, as shown in FIGS. 16 and 17.
However, in the impacter 121, it is necessary that the introduction portion 122 is exposed to the open air. Accordingly, in the case that the impacter 121 mentioned above is employed in the inflow portion of the sample gas S in the PM concentration measuring apparatus 120, it is necessary to place the PM concentration measuring apparatus 120 in a state in which the PM concentration measuring apparatus 120 is held in a support stand 125 having such durability as to be capable of being placed in the open air, as shown in FIG. 16. Or, in the case that the PM concentration measuring apparatus 120 is arranged in the room, it is necessary that the impacter 121 is exposed to the open air by piercing holes for mounting the impacter 121 on a ceiling portion, or by mounting the impacter 121 to an existing atmospheric air introduction portion 102a as shown in FIG. 15.
In any event, it is necessary to perform the mounting which may require a significant cost and time for the purpose of changing the specification of the sizing device, delaying spreading of the sampling of the PM as mentioned in the “Preliminary Manual for Method of Measuring Mass Concentration of Minute Particulate Matter (PM2.5) in Atmospheric Air” above.
Further, as shown in FIG. 17, the introduction portion 122 of the conventional impacter 121 introduces the peripheral air as the sample gas S from all angles (360 degrees) in view of structure, and in order to sufficiently achieve the sizing performance of the sizing device main body 105, it is necessary to introduce the sample gas S evenly, to be taken in from the periphery into the sizing device main body 105 in a state of rectifying the sample gas S so as to change direction downward as shown by the arrow A by the guide body 123.
Therefore, it is difficult to mount a filter (for example, an HEPA filter or the like) for removing the PM in the introduction port portion 122 of the impacter 121. Further, it is difficult to carry out a base line test as a basic instruction (confirmation of noise) in a non-dust state corresponding to an important test for confirming a basic performance of the PM concentration measuring apparatus 120. Accordingly, in the case of carrying out the base line test, it is necessary to take the impacter 121 out from the PM concentration measuring apparatus 120, and directly connect the filter to the PM concentration measuring apparatus 120.